1. Field of the invention
The invention relates to a stress detection method in which a semiconductor sensor having an oscillating member is used to detect stresses generated in the oscillating member according to amounts of dynamic forces acting on the oscillating member, such as pressure and the acceleration of gravity and a stress detection apparatus used in the stress detection method.
2. Description of Related Art
It has been proposed in dynamic quantity detection to utilize pertinently prepared semiconductor sensors. Such a dynamic quantity detecting semiconductor sensor is equipped with a stress generating member, such as a laminated film member which generates a stress according to pressure as a dynamic quantity acting thereon, or a beam member which generates a stress according to the acceleration of gravity as a dynamic quantity acting thereon. The dynamic quantity is determined by comparing the stress generated in the stress generating member with a quantitative relation between dynamic quantity and stress. The most importance in the dynamic quantity detection in which the utilization is made of the semiconductor sensor is how precise the stress detection is.
One approach to detect stresses by the semiconductor sensors is to utilize changes in resistance of a piezo electric resistor, or changes in resonant oscillation frequency of a beam
In the case where the utilization is made of changes in resistance of such a piezo electric resistor, a laminated film of the piezo electric resistor formed as the stress generating member provides a resistance value changing according to its strain or distortions caused due to a compression stress or a tensile stress generated therein by dynamic force acting thereon, based on which the stress generated in the stress generating member is determined. In this instance, since a specific quantitative relation is established between resistance value of the piezo electric resistor film and stress generated in the stress generating member, the stress is known from the resistance value with reference to the quantitative relation.
There is, however, such a drawback in the technique that the piezo electric resistor shows a relatively significant change in resistance due to changes in ambient temperature and experiences significant changes in physical characteristics due to aging. Accordingly, the detected resistance includes a change in resistance due to distortion or strain in addition to changes in resistance due to changes in ambient temperature and aging, resulting in a significant error in stress detection.
While the change in resistance due to ambient temperature can be compensated, it is necessary to provide a compensation circuit in the stress detection apparatus and there has not been no effective approach of eliminating the changes in resistance due to aging until today.
In the case where the utilization is made of changes in resonant oscillation frequency, the stress generating member is provided in the form of a straddle mounted beam oscillator, of which a resonant oscillation frequency is detected during an oscillation caused by periodically changing external exciting force. The straddle mounted beam oscillator causes a resonant oscillation whose oscillation frequency depends upon the stress generated in the straddle mounted beam oscillator.
Letting f.sub.r and N be the resonant oscillation frequency and the stress when the straddle mounted beam oscillator causes a resonant oscillation, the following quantitative relation is established: ##EQU1## where .alpha. and .beta. are positive invariables. When the resonant oscillation frequency f.sub.r is known, the stress N is determined from the above quantitative relation.
The utilization of the stress generating member in the form of a straddle mounted beam oscillator has a constraint on the extent of properly detectable stresses. Together, the detection of increased stress encounters a lack of accuracy. Specifically, since there is the quantitative relation between the resonant oscillation frequency and stress of the straddle mounted beam oscillator given by the formula (1), the condition of N.gtoreq.-.alpha./.beta. must be always satisfied. This indicates that the smallest detectable stress is greater than -.alpha./.beta. and it is impossible to detect a relatively large compression stress, consequently. Further, since a change in resonant oscillation frequency becomes smaller with an increase in stress generated in the straddle mounted beam oscillator, the resolution of stress detection is deteriorated with an increase in stress, resulting in large errors in stress detection.
While the utilization of the semiconductor sensor enables to detect dynamic quantities from stresses generated in the stress generating member, various constraints must be imposed on the semiconductor sensor. For instance, the semiconductor sensor must be enclosed in a vacuum container so as to be isolated from air resistance acting on the stress generating member during practical stress detection. The necessity of vacuum container renders the stress detecting apparatus troublesome and expensive to be manufactured and causes aggravation of yield due to dispersion in the degree of vacuum.
Even when a sufficiently high degree of vacuum is achieved in the vacuum container for the semiconductor sensor, the vacuum container experiences deterioration in vacuum leakage with progress of time. In cases where the semiconductor sensor installed in such a vacuum container is used to monitor the safety of an apparatus to which the semiconductor sensor is attached, unexpected situations occurring due to deterioration in the degree of vacuum of the vacuum container puts the semiconductor sensor unreliable and brings about a reduction in the safety of the apparatus.
Even if the problem of air resistance and deteriorated resolution in stress detection have been settled, there is still another problem that, if a band-pass filter has a distribution of transmission factors relatively widely spreaded, components with different frequencies mix in during frequency analysis. This is because, on one hand, the utilization is made of amplitude spectrum strength of various different frequencies and, on the other hand, it is hard as a design matter to provide the filter with a spread in distribution of transmission factors as small as possible.